Ymir is a Titan Shifter and a dreamer residing in the highly damaged Attack on Titan loop.
Ymir is a tall, androgynous girl with short hair and freckles. In loop, she is typically seen wearing the clothes of her world's armed forces.
In her Titan form, Ymir resembles a squat, frog-like giant with razor sharp teeth and no lips.
Prior to the loops, Ymir was an aloof girl who primarily associated with Christa Lenz (the alias of Historia Reiss), due to being aware that Historia was the illegitimate child of a noble and hoping to use that to protect herself. Despite her cynicism though, Ymir does possess a soft side, having acted to prevent Historia from killing herself trying to save another soldier, and often claims that she and Historia are alike. This softness eventually manifested itself during the Siege of Castle Utgard, where several Titans threatened the lives of Historia, Connie Springer, Reiner Braun and Bertolt Hoover. During the siege, Ymir voluntarily revealed herself to be a Titan Shifter and saved the lives of her friends, despite the danger to herself. She is currently in a relationship with Historia.
Beyond the events of the average run of the loop though, Ymir's history has been lost. Nothing is known about her, bar a handful of tacit notes: she is somehow important to the Titans, was shunned for some unknown reason, and was aware of the real reason the Titans existed. Even Ymir cannot provide an accurate history for herself, as the details and broad events of her past change on a loop to loop basis.
It was due to her convoluted history that Ymir has yet to begin conventionally looping. In her stead, her Titan form was gifted an artificial intelligence named Blaine that allowed it to loop by proxy. These factors were enough to evoke the Dreaming Glitch in Ymir, allowing her to recall (with relative but not perfect clarity) the events of previous loops. It is also due to the AI that Ymir typically abstains from using her Titan form.
Titan Shifting: Ymir's primary power is her ability to become a squat, but very agile Titan. This gifts her regeneration in both human and Titan forms, and perhaps some nascent psychic powers.
Peak Human Conditioning: Ymir's physique is impressive, owing to her size and regenerative capabilities. She is notably taller than most of the over loopers.
Historia Reiss (Attack on Titan): Ymir's girlfriend, the two of them met the first day of Basic Training, though it is likely that Ymir deliberately set up the meeting to get in Historia's good graces. However, the two later bonded over their shared histories as outcasts and pariahs, with Ymir admitting that she looked up to Historia's willingness to help others, even if it was a sham for some time. Historia in turn has stated that Ymir's fearless pride was what drove her to defy her father's wishes. On this basis, the two have gone into a relationship.
Armin Arlert (Attack on Titan): Armin and Ymir's relationship is... complicated. Though neither were particularly close in baseline, an early variant loop incited a mother-son relationship between Armin and Historia, which eventually translated to the looping Historia. In turn, Ymir has become a sort of confidant/friend/father-figure to Armin, of a sort.
Jean Kirstein (Attack on Titan): He owes her money, and she will forever lord that fact over his head.
Mikasa Ackerman (Attack on Titan): She respects Mikasa for her devotion to Eren... but is not above trolling her.